The present invention relates to a method and apparatus for manufacturing a solar cell and, in particular, to controlling nitrogen doping levels in an absorber layer of a Cu2ZnSn(S,Se)4 (CZTSSe) solar cell.
A solar cell includes multiple layers of material, with each layer having a specific function with respect to operation of the solar cell. For example, the absorber layer of the solar cell is the light sensitive layer which captures light from the sun and creates electron-hole pairs which, if collected, produce an electrical current. Among other selection criteria, the absorber layer must have an appropriate doping density in order to achieve an optimal p-n junction (i.e. the junction which produces a built-in voltage in the device) when interfaced with the buffer layer. The doping density in the absorber is one parameter which can impact the voltage produced by the solar cell. An emerging material known as Cu2ZnSn(S,Se)4 (also known as CZTSSe) has been shown to be suitable for use as the absorber layer of a solar cell. Typical preparation methods for CZTSSe rely on intrinsic point defects in the material to produce the desirable doping density described above. However, the nature of intrinsic doping in CZTSSe is not well understood and is commonly difficult to control. Nitrogen has the potential to impact the doping density in CZTSSe. Therefore, it is of interest to find methods for incorporating nitrogen uniformly into CZTSSe.